use the new autosign API on Foreman 1.15

[evgeni]

http://projects.theforeman.org/issues/7495

we can at least try to add an autosign entry before yelling at the user ;)

[sideangleside]

👍 So we'd effectively try to create the autosign entry (and gracefully handle if we can't - such as on older versions of Foreman?)